Setting and hardening behavior of volcanic ash phosphate cement

Abstract

This study aims at determining the setting and hardening behavior of volcanic ash-phosphate cement (VAPC) at room temperature using various concentrations of phosphoric acid. The properties of the fresh and hardened pastes of the VAPCs were assessed. The results reveal that iron is the main element taking part in the hardening reaction followed by aluminum, calcium, and magnesium, while silicon remained inert in phosphoric acid. The initial setting time is delayed when the concentration of phosphoric acid is increased (low pH). The low solubility of calcium and magnesium at high pH fostered the rapid formation of the binder, which hinders the further dissolution of the remaining particles. Whilst the high solubility of iron and aluminum at low pH slows down the rate of reaction but is beneficial for the extensive dissolution of reactive elements leading to the formation of a binder with high compressive strength. The microstructural analyses revealed that the binder is X-ray amorphous with a homogeneous distribution of all elements except silicon which is present in very few amounts.